diff --git a/docs-site/src/content/docs/ja/reference/adapters.md b/docs-site/src/content/docs/ja/reference/adapters.md
index 0bd13fbc5..1bf9f9bcf 100644
--- a/docs-site/src/content/docs/ja/reference/adapters.md
+++ b/docs-site/src/content/docs/ja/reference/adapters.md
@@ -74,7 +74,7 @@ HTTP リトライ ループの対象外です。
 
 - システムプロンプト → `systemInstruction`；メッセージ → `contents[]`（assistant → `model`）；ツール →
   `functionDeclarations`。data URL 画像 → `inline_data`。
-- Gemini が tool-call id を省略すると合成します。Antigravity では実際の `thoughtSignature` 値を保存・再利用し、次のターンでも reasoning の連続性を保ちます。
+- Gemini が tool-call id を省略すると合成します。Vertex と Antigravity では実際の `thoughtSignature` 値を保存・再利用し、tool-result の継続ターンでも reasoning の連続性を保ちます。
 
 ## `kiro`
 
diff --git a/docs-site/src/content/docs/ko/reference/adapters.md b/docs-site/src/content/docs/ko/reference/adapters.md
index bddf944b7..0ed83cc09 100644
--- a/docs-site/src/content/docs/ko/reference/adapters.md
+++ b/docs-site/src/content/docs/ko/reference/adapters.md
@@ -86,8 +86,8 @@ interface ProviderAdapter {
 
 - 시스템 프롬프트 → `systemInstruction`; 메시지 → `contents[]`(assistant → `model`); 툴 →
   `functionDeclarations`. data URL 이미지 → `inline_data`.
-- Gemini가 tool-call id를 생략하면 합성합니다. Antigravity에서는 실제 `thoughtSignature` 값을
-  보존하고 재사용해 다음 턴에서도 reasoning 연속성을 유지합니다.
+- Gemini가 tool-call id를 생략하면 합성합니다. Vertex와 Antigravity에서는 실제
+  `thoughtSignature` 값을 보존하고 재사용해 tool-result 후속 턴에서도 reasoning 연속성을 유지합니다.
 
 ## `kiro`
 
diff --git a/docs-site/src/content/docs/reference/adapters.md b/docs-site/src/content/docs/reference/adapters.md
index dd6224d6c..7119ba7d7 100644
--- a/docs-site/src/content/docs/reference/adapters.md
+++ b/docs-site/src/content/docs/reference/adapters.md
@@ -90,8 +90,8 @@ of the HTTP retry loop.
 
 - System prompt → `systemInstruction`; messages → `contents[]` (assistant → `model`); tools →
   `functionDeclarations`. Data-URL images → `inline_data`.
-- Tool-call ids are synthesized when Gemini omits them. Antigravity preserves and replays real
-  `thoughtSignature` values so reasoning continuity survives later turns.
+- Tool-call ids are synthesized when Gemini omits them. Vertex and Antigravity preserve and replay
+  real `thoughtSignature` values so tool-result continuations retain Gemini reasoning continuity.
 - **Inline image output:** when the model is one of the explicit image-capable chat IDs
   (`gemini-3.1-flash-image`, `gemini-2.0-flash-preview-image-generation`, or
   `gemini-3-pro-image-preview`), the adapter sends `responseModalities: ["TEXT", "IMAGE"]`.
diff --git a/docs-site/src/content/docs/ru/reference/adapters.md b/docs-site/src/content/docs/ru/reference/adapters.md
index d84c86693..a1f3950fa 100644
--- a/docs-site/src/content/docs/ru/reference/adapters.md
+++ b/docs-site/src/content/docs/ru/reference/adapters.md
@@ -91,9 +91,9 @@ interface ProviderAdapter {
 
 - Системный промпт → `systemInstruction`; сообщения → `contents[]` (assistant → `model`);
   инструменты → `functionDeclarations`. Изображения из data-URL → `inline_data`.
-- Идентификаторы вызовов инструментов синтезируются, когда Gemini их опускает. Antigravity
-  сохраняет и повторно передаёт настоящие значения `thoughtSignature`, чтобы непрерывность
-  рассуждений сохранялась в последующих ходах.
+- Идентификаторы вызовов инструментов синтезируются, когда Gemini их опускает. Vertex и Antigravity
+  сохраняют и повторно передают настоящие значения `thoughtSignature`, чтобы непрерывность
+  рассуждений сохранялась после возврата результата инструмента.
 
 ## `kiro`
 
diff --git a/docs-site/src/content/docs/zh-cn/reference/adapters.md b/docs-site/src/content/docs/zh-cn/reference/adapters.md
index 032df8806..9bde3ab29 100644
--- a/docs-site/src/content/docs/zh-cn/reference/adapters.md
+++ b/docs-site/src/content/docs/zh-cn/reference/adapters.md
@@ -80,8 +80,8 @@ interface ProviderAdapter {
 
 - 系统提示词 → `systemInstruction`；消息 → `contents[]`（assistant → `model`）；工具 →
   `functionDeclarations`；data URL 图像 → `inline_data`。
-- Gemini 省略 tool-call id 时会合成 id。Antigravity 会保留并重放真实 `thoughtSignature`，使
-  reasoning continuity 延续到后续 turn。
+- Gemini 省略 tool-call id 时会合成 id。Vertex 与 Antigravity 会保留并重放真实
+  `thoughtSignature`，使 tool-result 后续 turn 保持 reasoning continuity。
 
 ## `kiro`
 
diff --git a/src/adapters/google-antigravity-replay.ts b/src/adapters/google-antigravity-replay.ts
index 319f9cbb6..0334b930a 100644
--- a/src/adapters/google-antigravity-replay.ts
+++ b/src/adapters/google-antigravity-replay.ts
@@ -2,14 +2,15 @@ import { createHash } from "node:crypto";
 import { enforceAppOwnedMemoryBudget } from "../lib/app-owned-memory";
 
 /**
- * Antigravity (Cloud Code Assist) thoughtSignature reasoning-replay cache.
+ * Google-family thoughtSignature reasoning-replay cache.
  *
  * Gemini-3 interleaved thinking is stateless upstream: each model content part carries a
  * `thoughtSignature` that MUST be echoed back on the matching part in the next request, or the
  * upstream rejects the turn (HTTP 400). We observe signatures on the response stream, cache them
  * per `model + session`, and re-inject them into the outgoing `request.contents` on the next turn.
  *
- * Mirrors CLIProxyAPI `internal/runtime/executor/antigravity_reasoning_replay.go`. Gemini-only;
+ * Mirrors CLIProxyAPI `internal/runtime/executor/antigravity_reasoning_replay.go` and is also used
+ * by Vertex with a transport/project/location-prefixed model identity. Gemini-only;
  * Claude-on-Antigravity uses inline signature sanitization instead (see google-antigravity-wire).
  */
 
diff --git a/src/adapters/google.ts b/src/adapters/google.ts
index 9f97b6ac1..57c9a2ce3 100644
--- a/src/adapters/google.ts
+++ b/src/adapters/google.ts
@@ -51,6 +51,13 @@ function resolveVertexApiKey(optKey?: string): string | undefined {
   return realKey || process.env.GOOGLE_CLOUD_API_KEY;
 }
 
+/** Prefer Codex's stable opaque thread key; retain the existing deterministic fallback for clients
+ * that omit it. The replay store hashes this value and never retains the raw session identifier. */
+function vertexReplaySessionId(parsed: OcxParsedRequest): string {
+  const promptCacheKey = parsed.options.promptCacheKey?.trim();
+  return promptCacheKey || antigravitySessionId(parsed);
+}
+
 /**
  * Stable tool-call id for the Gemini wire `functionCall.id` / `functionResponse.id` fields.
  *
@@ -302,6 +309,11 @@ export function createGoogleAdapter(provider: OcxProviderConfig): ProviderAdapte
   // can stash the CCA model/session for parseStream's reasoning-replay observation.
   let antigravityModel: string | undefined;
   let antigravitySession: string | undefined;
+  // Vertex returns the same opaque Gemini thought signatures as CCA, but its replay namespace
+  // must stay transport-scoped: a signature minted by one Google backend must never be sent to
+  // another merely because the public model id and first prompt happen to match.
+  let vertexReplayModel: string | undefined;
+  let vertexReplaySession: string | undefined;
   let restoreGoogleToolName = (name: string): string => name;
   return {
     name: "google",
@@ -436,6 +448,20 @@ export function createGoogleAdapter(provider: OcxProviderConfig): ProviderAdapte
       if (provider.googleMode === "vertex") {
         const compiled = compileGoogleWireBody(body);
         restoreGoogleToolName = compiled.restoreToolName;
+        const vertexProject = provider.project || process.env.GOOGLE_CLOUD_PROJECT || process.env.GCLOUD_PROJECT || "api-key";
+        const vertexLocation = provider.location || process.env.GOOGLE_CLOUD_LOCATION || "global";
+        vertexReplayModel = `vertex:${vertexProject}:${vertexLocation}:${parsed.modelId}`;
+        vertexReplaySession = vertexReplaySessionId(parsed);
+        // Compile names before replay so the cache matches the exact provider-visible
+        // functionCall identity. This is the same bounded TTL/LRU store used by CCA, with the
+        // transport prefix above preventing cross-backend signature reuse (#1254).
+        if (Array.isArray((compiled.body as { contents?: unknown[] }).contents)) {
+          applyAntigravityReplay(
+            vertexReplayModel,
+            vertexReplaySession,
+            (compiled.body as { contents: unknown[] }).contents,
+          );
+        }
         // Vertex AI: project/location endpoint with GCP ADC, or x-goog-api-key fast path.
         const apiKey = resolveVertexApiKey(provider.apiKey);
         if (apiKey) {
@@ -511,9 +537,12 @@ export function createGoogleAdapter(provider: OcxProviderConfig): ProviderAdapte
           const err = chunk.error as { message?: string } | undefined;
           // Clear-on-invalid: a signature rejection means our replayed thoughtSignatures are stale.
           // Drop the cache entry so the next turn starts clean instead of re-injecting a bad sig.
-          if (provider.googleMode === "cloud-code-assist" && antigravityModel && antigravitySession
+          const replayModel = provider.googleMode === "cloud-code-assist" ? antigravityModel : vertexReplayModel;
+          const replaySession = provider.googleMode === "cloud-code-assist" ? antigravitySession : vertexReplaySession;
+          if ((provider.googleMode === "cloud-code-assist" || provider.googleMode === "vertex")
+            && replayModel && replaySession
             && /signature|invalid_argument|invalid argument/i.test(err?.message ?? "")) {
-            clearAntigravityReplay(antigravityModel, antigravitySession);
+            clearAntigravityReplay(replayModel, replaySession);
           }
           yield { type: "error", message: err?.message ?? "upstream error" };
           return "terminate";
@@ -547,9 +576,13 @@ export function createGoogleAdapter(provider: OcxProviderConfig): ProviderAdapte
         }
 
         const parts = candidates[0].content?.parts as { text?: string; functionCall?: { name: string; args: unknown } }[] | undefined;
-        // Antigravity reasoning-replay: record thoughtSignatures from the model parts for the next turn.
-        if (provider.googleMode === "cloud-code-assist" && parts && antigravityModel && antigravitySession) {
-          observeAntigravityReplay(antigravityModel, antigravitySession, parts as unknown[]);
+        // Record Gemini thought signatures for the next stateless tool-result turn. Vertex and
+        // Antigravity use separate model namespaces so opaque provider state cannot cross routes.
+        const replayModel = provider.googleMode === "cloud-code-assist" ? antigravityModel : vertexReplayModel;
+        const replaySession = provider.googleMode === "cloud-code-assist" ? antigravitySession : vertexReplaySession;
+        if ((provider.googleMode === "cloud-code-assist" || provider.googleMode === "vertex")
+          && parts && replayModel && replaySession) {
+          observeAntigravityReplay(replayModel, replaySession, parts as unknown[]);
         }
         if (parts) {
           for (const part of parts) {
@@ -764,9 +797,13 @@ export function createGoogleAdapter(provider: OcxProviderConfig): ProviderAdapte
       let toolCallsStarted = 0;
       const imageBudget = createImageBudget();
       if (candidates?.[0]?.content?.parts) {
-        // Non-streaming CCA: observe thoughtSignatures for the next turn, same as the stream path.
-        if (provider.googleMode === "cloud-code-assist" && antigravityModel && antigravitySession) {
-          observeAntigravityReplay(antigravityModel, antigravitySession, candidates[0].content.parts as unknown[]);
+        // Non-streaming Google-family response: observe thought signatures for the next turn,
+        // using the same transport-scoped namespace as the streaming path.
+        const replayModel = provider.googleMode === "cloud-code-assist" ? antigravityModel : vertexReplayModel;
+        const replaySession = provider.googleMode === "cloud-code-assist" ? antigravitySession : vertexReplaySession;
+        if ((provider.googleMode === "cloud-code-assist" || provider.googleMode === "vertex")
+          && replayModel && replaySession) {
+          observeAntigravityReplay(replayModel, replaySession, candidates[0].content.parts as unknown[]);
         }
         for (const part of candidates[0].content.parts) {
           if (part.text) events.push({ type: "text_delta", text: part.text });
diff --git a/structure/04_transports-and-sidecars.md b/structure/04_transports-and-sidecars.md
index 55f8c5a2c..359ba7383 100644
--- a/structure/04_transports-and-sidecars.md
+++ b/structure/04_transports-and-sidecars.md
@@ -382,6 +382,26 @@ pre-compaction checkpoint is not persisted for later carry-forward.
 - 장점, 단점 및 영향: Active-context reporting stays monotonic within an uncompacted Cursor conversation; no-checkpoint turns remain estimated; a process restart loses the numeric cache, and when neither a checkpoint nor a carry-forward is available the turn reports a request-local estimate derived from the same pruned payload sent to Cursor (#373 — reporting output-only usage made Codex read the context as nearly empty). Estimates are never persisted or promoted into checkpoint carry-forward; only live checkpoint frames update the cache.
 ```
 
+## Google tool-call thought-signature replay
+
+Gemini may attach an opaque `thoughtSignature` to a `functionCall` and requires that exact value on
+the matching model turn when its tool result is submitted. Antigravity and Vertex share the existing
+bounded TTL/LRU replay store, keyed by compiled function-call name plus canonical arguments. Vertex
+prefixes its cache model key with the transport, project, and location identity, so a signature
+minted by Vertex cannot be sent to Antigravity even when both routes expose the same public model id.
+Vertex prefers Codex's opaque `prompt_cache_key` for session identity and falls back to the existing
+first-user-message derivation for clients that omit it; only the fixed hash is retained.
+Both streaming and non-streaming responses feed the store; request compilation happens before replay
+so matching uses the provider-visible tool name.
+
+[Decision Log]
+- 목적과 의도: Preserve Vertex Gemini tool-call continuation without exposing opaque signatures to Codex or another Google backend.
+- 기존 구현 및 제약 조건: Responses history does not carry a safe Gemini signature field; Antigravity already used a bounded in-process replay cache, while Vertex bypassed it and received HTTP 400 after the first tool call.
+- 검토한 주요 대안: Serialize the signature into Responses item ids or reasoning content; create an unbounded Vertex map; reuse the bounded cache with or without a transport namespace.
+- 선택한 방식: Reuse the bounded cache for Vertex, observe both response shapes, apply after wire-name compilation, and scope Vertex by transport/project/location plus the opaque client session key when available.
+- 다른 대안 대신 이 방식을 선택한 이유: Responses ids are not Gemini signatures and previously caused Base64/TYPE_BYTES failures; a second cache duplicates limits; an unscoped cache could send provider-private state across destinations.
+- 장점, 단점 및 영향: Tool loops continue with exact opaque state and bounded memory while cross-transport reuse fails closed. Replay remains process-local, matching the existing Antigravity contract.
+
 ## OpenRouter provider routing
 
 The canonical OpenRouter `openai-chat` transport may carry optional provider-routing preferences
diff --git a/tests/google-vertex-thought-signature.test.ts b/tests/google-vertex-thought-signature.test.ts
new file mode 100644
index 000000000..f24a33f7c
--- /dev/null
+++ b/tests/google-vertex-thought-signature.test.ts
@@ -0,0 +1,123 @@
+import { beforeEach, describe, expect, test } from "bun:test";
+import { createGoogleAdapter as createGoogleAdapterProduction } from "../src/adapters/google";
+import {
+  __resetAntigravityReplayCache,
+  applyAntigravityReplay,
+} from "../src/adapters/google-antigravity-replay";
+import { antigravitySessionId } from "../src/adapters/google-antigravity-wire";
+import type { AdapterEvent, OcxParsedRequest, OcxProviderConfig } from "../src/types";
+import { withTestTranslatorBudget } from "./helpers/translator-budget";
+
+const createGoogleAdapter = (...args: Parameters<typeof createGoogleAdapterProduction>) =>
+  withTestTranslatorBudget(createGoogleAdapterProduction(...args));
+
+const SIGNATURE = "CiQAx-vertex-thought-signature-0123456789abcdef";
+const MODEL = "gemini-3.6-flash";
+
+const provider = {
+  adapter: "google",
+  googleMode: "vertex",
+  baseUrl: "https://aiplatform.googleapis.com",
+  apiKey: "vertex-test-key",
+} as OcxProviderConfig;
+
+function request(messages: OcxParsedRequest["context"]["messages"], stream: boolean): OcxParsedRequest {
+  return {
+    modelId: MODEL,
+    stream,
+    context: {
+      messages,
+      systemPrompt: [],
+      tools: [{ name: "shell_command", description: "run a command", parameters: { type: "object" } }],
+    },
+    options: {},
+  } as unknown as OcxParsedRequest;
+}
+
+const firstTurn = (stream: boolean) => request([{ role: "user", content: "run pwd" }], stream);
+
+const continuation = () => request([
+  { role: "user", content: "run pwd" },
+  {
+    role: "assistant",
+    content: [{
+      type: "toolCall",
+      id: "call_shell_1",
+      name: "shell_command",
+      arguments: { command: "pwd" },
+    }],
+  },
+  {
+    role: "toolResult",
+    toolCallId: "call_shell_1",
+    toolName: "shell_command",
+    content: "/workspace",
+  },
+], false);
+
+function vertexResponseBody(): Record<string, unknown> {
+  return {
+    candidates: [{
+      content: {
+        role: "model",
+        parts: [{
+          functionCall: { name: "shell_command", args: { command: "pwd" } },
+          thoughtSignature: SIGNATURE,
+        }],
+      },
+      finishReason: "STOP",
+    }],
+    usageMetadata: { promptTokenCount: 5, candidatesTokenCount: 2 },
+  };
+}
+
+function replayedFunctionCall(body: string): Record<string, unknown> {
+  const parsed = JSON.parse(body) as { contents: Array<{ role?: string; parts?: Record<string, unknown>[] }> };
+  const model = parsed.contents.find(content => content.role === "model");
+  const part = model?.parts?.find(candidate => "functionCall" in candidate);
+  if (!part) throw new Error("compiled Vertex request omitted the replayed functionCall");
+  return part;
+}
+
+describe("Vertex thought-signature continuation (#1254)", () => {
+  beforeEach(() => __resetAntigravityReplayCache());
+
+  test("streaming functionCall signature is replayed on the next tool-result turn", async () => {
+    const firstAdapter = createGoogleAdapter(provider);
+    await firstAdapter.buildRequest(firstTurn(true));
+    const response = new Response(`data: ${JSON.stringify(vertexResponseBody())}\n\n`, {
+      headers: { "content-type": "text/event-stream" },
+    });
+    const events: AdapterEvent[] = [];
+    for await (const event of firstAdapter.parseStream(response)) events.push(event);
+    expect(events.some(event => event.type === "tool_call_start")).toBe(true);
+    expect(events.at(-1)?.type).toBe("done");
+
+    const followup = await createGoogleAdapter(provider).buildRequest(continuation());
+    expect(replayedFunctionCall(followup.body as string).thoughtSignature).toBe(SIGNATURE);
+  });
+
+  test("non-streaming functionCall signature is replayed unchanged", async () => {
+    const firstAdapter = createGoogleAdapter(provider);
+    await firstAdapter.buildRequest(firstTurn(false));
+    const events = await firstAdapter.parseResponse!(new Response(JSON.stringify(vertexResponseBody())));
+    expect(events.some(event => event.type === "tool_call_start")).toBe(true);
+
+    const followup = await createGoogleAdapter(provider).buildRequest(continuation());
+    expect(replayedFunctionCall(followup.body as string).thoughtSignature).toBe(SIGNATURE);
+  });
+
+  test("Vertex signatures cannot enter the Antigravity replay namespace", async () => {
+    const first = firstTurn(false);
+    const adapter = createGoogleAdapter(provider);
+    await adapter.buildRequest(first);
+    await adapter.parseResponse!(new Response(JSON.stringify(vertexResponseBody())));
+
+    const contents = [{
+      role: "model",
+      parts: [{ functionCall: { name: "shell_command", args: { command: "pwd" } } }],
+    }];
+    applyAntigravityReplay(MODEL, antigravitySessionId(first), contents);
+    expect((contents[0].parts[0] as { thoughtSignature?: string }).thoughtSignature).toBeUndefined();
+  });
+});